In a known GNSS there are typically a plurality of satellites which broadcast positioning signals, said signals being timed using highly accurate clocks aboard the satellites where all the clocks of the satellites pertaining to a specific constellation are accurately synchronized.
The basic idea underlying such positioning systems is that a mobile user on or close to earth (such as an automobile or an aircraft) with respect to GNSS satellite orbit altitude, capable of receiving directly or indirectly through any kind of retransmission of said signals can thus determine its position by an accurate measurement of the time a signal has taken to travel from the a given satellite to its receiver, assuming that the location of the satellite is determined, which is the case in practice. Usually the mobile user receives signals from a group of satellites in order determine its exact coordinates and local time.
The signals received from the satellites may be subject to inaccuracies which are undesirable in a global positioning system. Some of the most commonly known inaccuracies, or errors, in the satellite signals are caused by the so-called ionospheric and tropospheric delays due to relatively rapid variations in the status of these atmospheric layers. Another error is the so-called ephemeris error which is the difference between the data indicating a satellite's real location and the data on the location it is expected to be according to calculations, a similar may be caused in relation to the real and calculated timing of the satellite. Another error is caused by multipath propagation of the signal. Finally there may be errors introduced deliberately by government authorities in the data transmitted by the satellite for security considerations.
As these errors can adversely affect the accuracy, integrity, continuity and availability of the global positioning systems, their elimination or at least their reduction to practically desirable levels has become a need. Various solutions have been proposed for overcoming such drawbacks.
According to a known solution, a monitoring ground station is used to determine the level of error of the satellites transmitting positioning signals. In order to determine the error, said monitoring ground station is required to be in an accurately known position. The monitoring ground station receives signals broadcast from positioning satellites which enable it to determine its position. This data is then compared at one or more navigation computing centers to the real position of the monitoring ground station and by such comparison the level of error is determined. This information, combined with information coming from other monitoring ground stations, is indicative of the amount of the correction needed to be applied to a given satellite signal. The resulting navigation correction data is then transmitted to the mobile user, together with a guaranteed level of reliability and confidence (integrity information) which may be expressed in a so called uncertainty radius for the three-dimensional position of the mobile user, as well as for the time and status of the GNSS satellite fleet. One known difficulty is that acquiring all satellite data needed to compute a navigation solution which would enable the determination of a precise location and time requires acquiring GNSS satellite fleet predicted position through the so-called GNSS almanacs which are sent at relatively low rate, thus causing a receiving position to need significant time such as typically 15 min to start evaluating mobile position. In case of a wide area application, the correction information is sent to a geo-stationary satellite and through the latter it is broadcast to all mobile users under its coverage. Often the geo-stationary satellite is also used for transmitting a signal that appears as it were directly coming from a GNSS satellite, thus adding more capability in locations where GNSS coverage is poor.
This solution typically requires that each satellite used for this purpose have a navigation specific payload comprising at least a transponder integrated therein. This requirement is costly and furthermore, its implementation would be limited by the payload capacity of the satellite. On the other hand, a number of applications exist where using a channel not pretending to look like a GNSS satellite channel may still be of interest.
Satellite based augmentation systems presently known, such as the European Geo-stationary Navigation Overlay Service (EGNOS) and Wide Area Augmentation System (WMS) use transponders in the L-Band. This band is in turn comprised of two frequency spectrums currently in use, namely the L1 band of 1575.42 MHz and the L2 band of 1227.6 MHz although the use of other frequencies are presently under development. The transponders are on-board specific satellites such a for example the INMARSAT communication satellites or the European Space Agency ARTEMIS experimental satellite. The navigation correction signals are sent to these specific satellites which provide a link to the user using said transponders. The signal timing is based on bursts of 1 Hz messages; however, the available bit rate in these messages, when said messages are compliant with the standard DO-229 is 500 bps only. This is a relatively slow rate giving rise to the fact that messages arrive too late to the destination. In fact, the total message of a computer center used in such augmentation systems has an actual user bit rate of 250 bits/sec, since half of the 500 bps are used for forward error correction due to the fact that the transmission needs very low bit error rate (BER), and that conventional navigation payloads do not have specific mechanisms to ensure it.
It is therefore desired to broadcast data relative to the correction of errors in navigation signals to a mobile user in a less costly manner, also maintaining the payload on-board a satellite as low as possible, while at the same time such data is transmitted at a higher speed. It is also be desired to improve redundancy at the user side by simultaneously broadcasting data computed in parallel by more than one computer center.